In control of a permanent magnet synchronous motor, vector control that can flexibly control torque is often used. A control apparatus employing the vector control adopts a configuration in which a PWM inverter decomposes a three-phase motor current, which is output to the permanent magnet synchronous motor, onto a dq-axis coordinate, i.e., rotating orthogonal two axes, and converts the three-phase motor current into a d-axis current as an excitation current component and a q-axis current as a torque contribution component. A current controller, which performs proportional integral control, controls the PWM inverter such that the converted d-axis current and q-axis current actually flowing to the permanent magnet synchronous motor follow a d-axis current command and a q-axis current command generated from a torque command externally given.
Therefore, accuracy of torque control by the control apparatus for the permanent magnet synchronous motor depends on whether an appropriate d-axis current command and an appropriate q-axis current command can be generated in a d/q-axis current command generator, which generates the d-axis current command and the q-axis current command from the torque command externally given.
A torque generation formula in an SPM (Surface Permanent Magnet) motor having no saliency, which is one of permanent magnet synchronous motors, is given by Formula (1). In Formula (1), Tm* represents the torque command externally given, iq* represents the q-axis current command, and Kt represents a torque constant of the permanent magnet synchronous motor.Tm*=Ktiq*  (1)
If Formula (1) is transformed as in the following Formula (2) and the d/q-axis current command generator is configured to carry out a calculation based on Formula (2), it is possible to control torque. In Formula (2), id* represents the d-axis current command.
                                                                                          i                  d                  *                                =                0                                                                                                          i                  q                  *                                =                                                      T                    m                    *                                    /                                      K                    t                                                                                      }                            (        2        )            
A torque generation formula in an IPM (Interior Permanent Magnet) motor having saliency, which is one of permanent magnet synchronous motors, is given by the following Formula (3). In Formula (3), Pm, Ld, and Lq respectively represent the number of pole pairs, d-axis inductance, and q-axis inductance of the permanent magnet synchronous motor.Tm*=Ktiq*+Pm(Ld−Lq)id*iq*  (3)
For the IPM motor, if the d/q-axis current command generator is configured to carry out a calculation based on Formula (3) or refer to table data based on Formula (3) prepared in advance, it is possible to control torque. Because it is generally known that the d-axis inductance and the q-axis inductance nonlinearly change according to the magnitude of an electric current, it is also possible to improve the accuracy of the torque control by configuring the d/q-axis current command generator taking that into account.
In recent years, to cause the permanent magnet synchronous motor to operate at high speed, operation in a constant output region where an inverter output voltage is fully consumed is increasing. In this operation, because it is necessary to suppress saturation of the inverter output voltage, so-called field weakening control for increasing the d-axis current in the negative direction is often performed as means for suppressing the saturation of the inverter output voltage.
A method of the field weakening control is described in Patent Document 1 (FIG. 11). If this method is applied to the control apparatus for the permanent magnet synchronous motor, the configuration will be as follows. A q-axis voltage saturation amount is calculated from a deviation between a q-axis voltage component and a q-axis voltage command and a d-axis current correction amount is calculated from the calculated q-axis voltage saturation amount and rotational angular velocity. A d-axis voltage saturation amount is calculated from a deviation between a d-axis voltage component and a d-axis voltage command and a q-axis current correction amount is calculated from the calculated d-axis voltage saturation amount and rotational angular velocity. Current commands on the d-axis and the q-axis output by the d/q-axis current command generator are corrected using the calculated current correction amounts on the d-axis and the q-axis. With this configuration, both of torque control in the permanent magnet synchronous motor and stable operation in a high-speed operation region can be achieved. Therefore, it is possible to suppress occurrence of voltage saturation during the high-speed operation, realize stable operation, and significantly improve stability of control.
Patent Document 1: International Publication No. 03/009463
Patent Document 2: Japanese Patent Application Laid-open No. 2000-116198